As one of the light beam-controlling techniques in the optical fiber communication system, a light source-optical fiber coupling technique to couple an output light from a semiconductor laser (LD) or light-emitting diode (LED) as a light source to a transmission path of optical fiber with high efficiency is important to increase the reliability of the communication circuit. In general, the output light from a light source is converged by means of an optical element such as a lens to improve its coupling efficiency to an optical fiber. As this optical element, a cylindrical lens, a spherical lens, a convergent rod lens having a gradient of refractive index, an ordinary non-spherical surface lens, or a combination thereof is considered to be effective.
Of these optical elements, the cylindrical lens is effective to converge a beam of light in one direction and, in particular, effective to collimate a beam of light having an emission angle varying with its direction as generated from a semiconductor laser light source. However, it is not possible for the cylindrical lens to completely collimate a beam of light because of its aberration. Furthermore, a non-spherical surface cylindrical lens is difficult to produce by polishing glass.
The present invention is intended to provide a slab-shaped lens having a gradient of refractive index in only one direction by utilizing a molecular stuffing method.
The molecular stuffing method is known as a method of preparing a rod-shaped glass product having a gradient of refractive index. In accordance with this method, a porous glass rod is used as a raw material and its fine pores are filled with dopants (refractive index modifying components). Japanese Patent Application (OPI) No. 126207/76 (the term "OPI" as used herein means a "published unexamined Japanese patent application") discloses, as well as a process for the production of porous glass, a process for the production of a glass product having a gradient of refractive index by utilizing the molecular stuffing method.
This process for the production of a glass product having a gradient of refractive index is described below.
A borosilicate glass is subjected to a heat treatment under predetermined conditions to separate it a SiO.sub.2 -rich phase and an alkali metal oxide and B.sub.2 O.sub.3 -rich phase. This alkali metal oxide and B.sub.2 O.sub.3 -rich phase is easily soluble in acids and thus leached by treating with an aqueous solution of an acid such as hydrochloric acid, sulfuric acid, and nitric acid to prepare a porous glass having continuous micropores with the SiO.sub.2 -rich phase as a skeleton. A solution of dopants is permeated into the micropores of the porous glass product as prepared above (stuffing) and then part of the dopants is leached out from the outside of the glass product (unstuffing) to form a gradient of dopant concentration in the glass product. Thereafter, the dopants are precipitated, and the glass product is dried and further subjected to a heat treatment to collapse the micropores, whereby the desired glass product having a gradient of refractive index in the direction of radius can be obtained. In the thus-produced rod-shaped glass product having a gradient of refractive index in the direction of radius, the gradient of refractive index varies according to the following equation (1): EQU n(r).sup.2 =n.sub.0.sup.2 [1-(gr).sup.2 ]
wherein r represents the radius, n.sub.0 represents the refractive index of the central portion, and g is a constant. Thus the glass product can be used as a rod-shaped lens for microlens arrays, or a microlens for coupling optical communication fibers.